Bioprecipitation of Calcite by Sporosarcina pasteurii: Developing Efficient Methodologies for Microbially Indurated Rammed Earth

Microbially-facilitated calcite precipitation has a high potential to reinforce unconsolidated soils, thus increasing their strength. Employing the use of microbially-facilitated precipitation of calcite will result in the creation of a new building material termed Microbially Indurated Rammed Earth (MIRE). This application of the bacterium S. pasteurii exploits the urease pathway to hydrolyze urea, resulting in the precipitation of calcite in Ca 2+-rich solutions. The resulting increase in strength derived from this process would meet standard building codes for residential structures in many states and could therefore supplement the building requirements of concrete. During the course of this study the maximum growth and optimal delivery method of a pure culture of the bacterium, Sporosarcina pasteurii was tested for its use as a natural binding agent. This study tested S. pasteurii for its cell density and growth rate using urea as a metabolite as studies have shown that bacterial cell concentration correlates to greater calcite precipitation. After ~70 hours, cell counts were taken to create growth curves for bacteria incubated at 35 °C and 25 °C. These data were applied to experiments on the bioprecipitation of calcite in a limestone soil as baseline parameters for the creation of MIRE. Over the course of this study it was determined that S. pasteurii grew most rapidly between 20-40 g L-1 urea at 35 °C. Additionally, there was no significant difference in the s patial distribution of bacteria, critical for equal distribution of calcite cement, when the bacteria were delivered to MIRE soil as a freeze-dried pellet compared to freshly grown and harvested. Pilot tests of RE using S. pasteurii with urea and blood were performed. Compressive strength tests were done on standard engineering cylinders in order to test the efficacy of soil stabilized using MICP. These data provide the groundwork for meeting RE construction standards and building codes.